This invention relates to circuits that reduce output voltage overshoot. More particularly, this invention relates to integrated circuits that reduce output voltage overshoot in opto-coupler controlled closed-loop DC power supplies.
An output voltage overshoot is a transient rise in output voltage beyond a specified output voltage level. Excessive overshoot can cause system failure and can damage both the power supply and loads coupled to the power supply. Output overshoot typically occurs when the power supply is first turned on or when the power supply output is overloaded or inadvertently shorted to ground or to a voltage less than the regulated output voltage (i.e., shorted to an “undervoltage”) and then released. In sum, the following three conditions can cause output voltage overshoot: start-up, output short to ground (hard short), and output short to an undervoltage (soft short).
Known closed-loop isolated power supplies include feedback circuitry that regulates the output voltage (i.e., maintains the output voltage at a specified level). Such feedback circuitry commonly includes a high gain amplifier and an opto-coupler. When the power supply output voltage rises above a threshold, the amplifier drives the opto-coupler, which provides one or more signals to a power supply control circuit that stops the rise in output voltage and allows the output voltage to return to its specified level. However, the response time of basic feedback circuitry is often slow. In particular, the slew time of the amplifier output is often longer than the slew time of the rising output voltage. Slew time is the time it takes a signal to make a transition. Thus, basic feedback circuitry is generally unable to prevent potentially large output voltage overshoots from occurring, thus only belatedly being able to regulate the overshot output voltage back down to the desired level.
Improved feedback circuits are also known. For example, soft-start circuits are known that limit or prevent overshoot at start-up. Soft-start circuits gradually apply power to the output to slow the rising output voltage. The feedback circuitry then has sufficient time to respond to the output voltage as it reaches the specified level. These circuits, however, are ineffective against soft and hard output shorts.
Feedforward slew rate detector circuits and overvoltage comparator circuits are also known. These circuits generate and route signals to control circuitry via a fast-path around the slower feedback amplifier path in response to the output voltage reaching a threshold. However, output voltage ripples, which typically occur on the output of DC power supplies, are often of sufficient magnitude to exceed thresholds used in the fast-paths of such circuits and can thus cause the power supply output to oscillate.
Clamping amplifier output circuits are also known in which the voltage swing on the feedback amplifier's output is limited in order to provide a quicker response to output overshoot. However, even the limited voltage swings of known clamping circuits are still too large to allow the circuit to respond quickly enough to prevent overshoot, thus resulting in only marginal improvement.
In sum, no known closed-loop power supply feedback circuit is effective against all three common overshoot conditions mentioned above.
In view of the foregoing, it would be desirable to be able to provide a circuit that reduces, if not eliminates, output voltage overshoot in an opto-coupler controlled closed-loop isolated power supply under any of several conditions.
It would also be desirable to be able to provide an integrated circuit that responds quickly to a threshold being met.